The invention generally relates to apparatuses and associated methods of operation whereby the operation of a fuel cell is coordinated with a fuel processing reactor and a hydrogen separation system.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
at the anode of the cell, and
O2+4H++4exe2x88x92xe2x86x922H2Oxe2x80x83xe2x80x83(2)
at the cathode of the cell.
A typical fuel cell has a terminal voltage of up to about one volt DC. For purposes of producing much larger voltages, multiple fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow field plates (graphite composite or metal plates, as examples) that are stacked one on top of the other. The plates may include various surface flow field channels and orifices to, as examples, route the reactants and products through the fuel cell stack. The flow field plates are generally molded, stamped or machined from materials including carbon composites, plastics and metal alloys. A PEM is sandwiched between each anode and cathode flow field plate. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to act as a gas diffusion media and in some cases to provide a support for the fuel cell catalysts. In this manner, reactant gases from each side of the PEM may pass along the flow field channels and diffuse through the GDLs to reach the PEM. The GDL""s generally comprise either a paper or cloth based on carbon fibers. The PEM and its adjacent pair of catalyst layers are often referred to as a membrane electrode assembly (MEA). An MEA sandwiched by adjacent GDL layers is often referred to as a membrane electrode unit (MEU), or also as an MEA. Common membrane materials include Nation(trademark), Gore Select(trademark), sulphonated fluorocarbon polymers, and other materials such as polybenzimidazole (PBI) and polyether ether ketone. Various suitable catalyst formulations are also known in the art, and are generally platinum-based.
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. Thus, a controller of the fuel cell system may monitor the output power of the stack and based on the monitored output power, estimate the fuel flow to satisfy the appropriate stoichiometric ratios. In this manner, the controller regulates the fuel processor to produce this flow, and in response to the controller detecting a change in the output power, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.
The fuel cell system may provide power to a load, such as a load that is formed from residential appliances and electrical devices that may be selectively turned on and off to vary the power that is demanded by the load. Thus, the load may not be constant, but rather the power that is consumed by the load may vary over time and abruptly change in steps. For example, if the fuel cell system provides power to a house, different appliances/electrical devices of the house may be turned on and off at different times to cause the load to vary in a stepwise fashion over time. Fuel cell systems adapted to accommodate variable loads are sometimes referred to as xe2x80x9cload followingxe2x80x9d systems.
Fuel cell systems generally include various sources of heat, such as from fuel processing systems, the fuel cell stack itself, exhaust gas oxidizers, etc. In particular, the exhaust from a fuel cell is generally oxidized to remove trace amounts of unreacted fuels before it is exhausted to ambient. Such exhaust is generally hot and saturated with water vapor from the fuel cell system and from combustion of combustible gas components in the exhaust. For a variety of reasons, it may be desirable to recover such heat from a fuel cell system. As examples of such systems in the prior art, the teachings of U.S. application Ser. Nos. 09/728,227 now U.S Pat. No. 6,551,733 and 09/727,921 now U.S. Pat. No. 6,370,878 are hereby incorporated by reference.
Hydrogen purification systems have also been used with fuel cell systems in various ways. For example, a hydrogen purification system can be used to filter a reformate stream to produce a pure hydrogen stream that can be stored or used by a fuel cell. Hydrogen purification systems have also been used to recover hydrogen from fuel cell system exhaust streams. In the context of this invention, a hydrogen purification system may also be referred to as a hydrogen separator, and in either case, such a system can refer to any of the various techniques known in the art for separating hydrogen from gas streams, including electrochemical separation and pressure swing adsorption systems. As examples of such systems in the prior art, the teachings of U.S. Pat. No. 6,280,865, and U.S. application Ser. Nos. 10/214,022, 10/213,798, and 10/214,019 are hereby incorporated by reference.
There is a continuing need for fuel cell system designs and improvements to coordinate the integrated operation of systems including the foregoing.
The invention provides fuel cell systems and associated methods of operation whereby application of a fuel cell is coordinated with a fuel processor and a hydrogen separator. In particular, the reactant supply system of the fuel cell is made less susceptible to supply pressure transients associated with operation of the hydrogen separation subsystem.
In one aspect, a fuel cell system includes a fuel processing reactor, a hydrogen separator, a fuel cell, and an oxidizer. The fuel processor is coupled to the fuel cell via a first flow path, and to the hydrogen separator via a second flow path. An exhaust port of the fuel cell is coupled to the oxidizer via a third flow path, and an exhaust port of the hydrogen separator is coupled to the oxidizer via a fourth flow path.
A first flow restricting means is located along the first flow path and is adapted to reduce the pressure of reformate provided to the fuel cell from the fuel processing reactor. A second flow restricting means is located along the second flow path and is adapted to reduce the pressure of reformate provided to the hydrogen separator from the fuel processing reactor.
The term xe2x80x9ccoupledxe2x80x9d is used to refer to any direct or indirect connection between two elements of the system. As an example, an indirect connection of two components may include connections to various other components between them. Also, in the context of the present invention, the term xe2x80x9cflow pathxe2x80x9d generally refers to any conduit or housing through which the flow of a process stream is guided in the system. In some cases, different flow paths can be partially coextensive, as in the case where a common conduit splits into two conduits.
For illustration purposes, the discussion provided herein focuses on PEM fuel cell systems. For example, systems under the invention may utilize a PEM fuel cell having an operating temperature less than 100xc2x0 C. Also, the fuel cell may form a portion of a fuel cell stack. It will be appreciated that the invention may also be used with other types of fuel cells, such as solid oxide, phosphoric acid, molten carbonate, etc.
In embodiments utilizing an electrochemical hydrogen separator, the electrical current used by the hydrogen separator can be supplied by the fuel cell, by a battery, or by some other source, such as a power grid. In some cases, a combined fuel cell and electrochemical hydrogen separation stack can be used, as described in U.S. Pat. No. 6,280,865, and U.S. application Ser. Nos. 10/214,022, 10/213,798, and 10/214,019.
As examples, the first and second flow restricting means can each be a valve. For example, proportional valves (a valve that can be opened to a variable extent) can be used or a modulated binary valve (a valve that is either fully open or fully closed) can also be used to achieve the same effect as a proportional valve by periodically opening and closing. Valves used with the present invention are preferably automatically controlled, but the invention is not intended to be limited by a specific valve design. As another example, the first and second flow restricting means can each be an orifice serving to inhibit flow to a desired extent. As another example, the first and second flow restricting means can each be a spring-biased pressure regulator or a dome-loaded pressure regulator, or any other type of device that can be used to restrict flow. The first and second flow restricting means can also be any combination of the foregoing devices.
In some embodiments, the fuel processing reactor has an outlet coupled to a conduit, wherein the conduit is coupled to the first and second flow paths. In other words, the first and second flow paths are partially coextensive. This conduit can be coupled to a bypass flow path, wherein the bypass flow path includes a valve, and wherein the bypass flow path is coupled to the oxidizer. The fuel exhaust of the stack is preferably coupled to the oxidizer, along with the exhaust of the hydrogen separator. In another embodiment, the purified hydrogen outlet of the hydrogen separator can also be coupled to the first flow path.
In some embodiments, a pressure sensor is provided in the first flow path. A controller is coupled to the pressure sensor and to the second flow restricting means, and is adapted to vary a flow output from the second pressure regulator in response to a signal from the pressure sensor. Some embodiments may further include a valve located in the second flow path between the second flow restricting means and the hydrogen separator. For example, the valve can be closed when the hydrogen separator is not being used to prevent any flow of reformate through the second flow path. In some cases, a controller can be coupled to this valve and to the first flow restricting means, wherein the controller is adapted to open and close the valve to regulate reformate flow to the hydrogen separator, and wherein the controller is adapted to modulate the first flow restricting means to vary the flow of reformate to the fuel cell.
One advantage of this design is that the reactant supply system of the fuel cell is made less susceptible to supply pressure transients associated with operation of the hydrogen separation subsystem. For example, when the fuel cell is supplying an electrical load, if the hydrogen separator is switched on, the resulting drain in reformate supply can cause the stack to be under-supplied with reactants to meet its load. In addition to affecting the stack""s ability to maintain a steady supply of power to the load, the stack can also be damaged if an excess load is allowed to drive a fuel cell to a negative voltage. This problem is solved under the present system since a backpressure is created upstream from the fuel cell that acts as a buffer to pressure transients resulting from operation of the hydrogen separator.
In another aspect, a method is provided of coordinating operation of a combined fuel processor, fuel cell and hydrogen purification system, including at least the following steps: (1) operating a fuel processing reactor to convert a hydrocarbon into reformate; (2) flowing reformate through a first pressure regulator to reduce the pressure of the reformate; (3) supplying reformate from the first pressure regulator to a fuel cell to generate electrical power; (4) flowing a portion of the reformate from the fuel processor to a second pressure regulator to reduce the pressure of the reformate while generating the electrical power with the fuel cell; and (5) supplying reformate from the second pressure regulator to the hydrogen purification system while generating the electrical power with the fuel cell.
Embodiments of such methods may further include supplying the electrical power to a load having a power requirement greater than the electrical power supplied by the fuel cell; and regulating the flow of reformate from the fuel processor to the hydrogen purification system to maintain a pressure drop across the first pressure regulator.
In another aspect, a method is provided of operating a fuel cell system, including at least the following steps: (1) flowing reformate from a fuel processor to a fuel cell to generate an electrical current; (2) restricting a pressure of the flow of reformate from the fuel processor to the fuel cell to create a backpressure of reformate; (3) supplying the electrical current to a load having a power requirement greater than a power supplied by the electrical current; (4) releasing a portion of the backpressure of reformate to a flow of reformate from the fuel processor to a hydrogen separator; and (5) regulating the flow of reformate from the fuel processor to the hydrogen separator to maintain the backpressure at a pressure at least as great as a pressure of the reformate flowed to the fuel cell.
In some embodiments, the step of restricting a pressure of the flow of reformate from the fuel processor to the fuel cell includes opening a proportional valve to adjust the flow of reformate to increase the electrical current supplied to the load. Embodiments may further include supplying hydrogen from the hydrogen separator to the fuel cell.
Embodiments of such methods can also include any of the features, design aspects, techniques and methods described herein, either alone or in combination. Advantages and other features of the invention will become apparent from the following description, drawing and claims.